Magnetic disc memories used in data processing systems have data recorded in binary coded form on concentric record tracks arranged on both faces of the disc. Generally, a data processing system includes a central unit for processing information (CPU) or data associated with peripheral members, such as printers, typewriters, magnetic tape units, disc memory units, etc.
In disc memories, there is associated with each disc face a single electro-magnetic transducer which can read data from and write data into only one track at a time. Therefore, to write or read data on a disc face, it is necessary for the transducer to be moved parallel to the disc face. To recover data recorded on the disc, the transducer is associated with electronic circuits which derive electrical pulses corresponding with recorded data that pass in front of the transducer. Electronic write circuits respond to electrical pulses to supply the transducer with data to be recorded on the part of the disc which passes in front of the transducer. The electrical pulses supplied to and read by the transducers generally have only two values, zero and one, and are usually referred to as logic signals or binary bits.
Data are distributed over a disc face in equal and adjacent sectors of a circle, so each sector includes two parts having different areas. The larger sector part contains recorded signal data to be handled by the central processing unit or peripheral devices, while the smaller sector part, termed a reference zone, carries track locating information. Generally, a disc face is divided into several tens of sectors. Tracks on the disc are assigned a serial number J, ranging from zero to (N-1) where N is the total number of record tracks and J is a whole number. The coded expression of a track serial number J is referred to as an address. Generally, the track serial numbers are recorded in binary code.
The present trend in disc memories is to increase data density. To increase data density, both the radial density, i.e., the number of recording tracks per unit length count along a disc radius, and longitudinal density, i.e., the number of bits per unit circumferential recording track length, are increased. As data density increases, the likelihood of faulty tracks on the disc increases so there is a non-zero probability of having faulty sectors.
Because of the non-zero probability of having faulty sectors, both faces of the disc are frequently provided with a number of additional tracks, termed "salvage" tracks which receive data that normally is contained in the faulty sectors. To avoid reading or writing data from or into a defective sector, the beginning of each track of a defective sector includes an information item which indicates whether or not the track is faulty, as well as an information item for the salvage track address, in the event of the track being faulty.
The prior art arrangement has the following consequences:
(1) the transducers must move from a faulty track to a salvage track, which is generally situated at the disc periphery; PA1 (2) the fault handling arrangement must include at least one memory in which is recorded a fault table containing information relating to the faulty tracks; and PA1 (3) the address information for the salvage track may be faulty. PA1 (1) the time required to access the salvage track, largely dependent on the time required for the transducer to move, is considerable compared with the time required for the magnetic transducer to transfer data when it is already positioned at the required track and sector; PA1 (2) a microprogram generally having a complicated algorithm is required to manage the table; PA1 (3) if the salvage track address is faulty, the salvage track cannot be found and the data can be neither written into or read from the disc; and PA1 (4) the salvage track address at the beginning of a track requires significant space on the disc surface.
These consequences give rise to the following disadvantages:
(A) a microprogram is a fixed sequence of microinstructions which, while being executed, joins the various basic functions, which, together, represent a given function of an automatic data processing machine.